The present invention relates to the control of a gas turbine power plant and, more particularly, to the control thereof when its primary control system must be shut down for maintenance or experiences a failure. The present invention is particularly suitable for use as a backup control for primary control systems which utilize a programmed digital computer.
Industrial gas turbines may have varied cycle, structural and aerodynamic designs for a wide variety of uses. For example, gas turbines may employ the simple, regenerative, steam injection or combined cycle in driving an electric generator to produce electric power. Further, in these varied uses the gas turbine may have one or more shafts and many other rotor, casing, support and combustion system structural features which can vary relatively widely among differently designed units. They may be aviation jet engines adapted for industrial service as described for example in an ASME paper entitled "The Pratt and Whitney Aircraft Jet Powered 121MW Electrical Peaking Unit" presented at the New York Meeting in November- December 1964.
Other gas turbine uses include drive applications for pipeline or process industry compressors and surface transportation units. An additional application of gas turbines is that which involves recovery of turbine exhaust heat energy in other apparatus such as electric power or industrial boilers or other heat transfer apparatus. More generally, the gas turbine air flow path may form a part of an overall process system in which the gas turbine is used as an energy source in the flow path.
Gas turbine electric power plants are usable in base load, mid-range load and peak load power system applications. Combined cycle plants are normally usable for the base or mid-range applications while the power plant which employs a gas turbine only as a generator drive typically is highly useful for peak load generation because of its relatively low investment cost. Although the heat rate for gas turbines is relatively high in relation to steam turbines, the investment savings for peak load application typically offsets the higher fuel cost factor. Another economic advantage for gas turbines is that power generation capacity can be added in relatively small blocks such as 25MW or 50MW as needed for expected system growth thereby avoiding excessive capital expenditure and excessive system reserve requirements. Further background on peaking generation can be obtained in articles such as "Peaking Generation" a Special Report of Electric Light and Power dated Nov., 1966.
Startup availability and low forced outage rates are particularly important for peak load power plant applications of gas turbines. Thus, reliable gas turbine startup and standby operations are particularly important for power system security and reliability.
In the operation of gas turbine apparatus and electric power plants, various kinds of controls have been employed. Relay-pneumatic type systems form a large part of the prior art, but have heretofore not provided the flexibility desired, particularly in terms of decision making. Furthermore, such prior art systems have been characterized by being specially designed for a given turbine plant, and accordingly are not adaptable to provide different optional features for the user. More recently, electronic controls of the analog type have been employed as perhaps represented by U.S. Pat. No. 3,520,133 entitled "Gas Turbine Control System" and issued on July 14, 1970 to A. Loft or by the control referred to in an article entitled "Speedtronic Control, Protection and Sequential System" and designated as GER-2461 in the General Electric Gas Turbine Reference Library. See also U.S. Pat. No. 3,662,545, which discloses a particular type of analog acceleration control circuit for a gas turbine and U.S. Pat. No. 3,340,883, relating to an analog acceleration, speed and load control system for a gas turbine. A wide variety of controls have been employed for aviation jet engines including electronic and computer controls as described, for example, in a March, 1968 ASME Paper presented by J. E. Bayati and R. M. Frazzini and entitled "Digatec" (Digital Gas Turbine Engine Control), an April, 1967 paper in the Journal of the Royal Aeronautical Society authored by E. S. Eccles and entitled "The Use of a Digital Computer for On-line Control of a Jet Engine", or a July, 1965 paper entitled "The Electronic Control of Gas Turbine Engines" by A. Sadler, S. Tweedy and P. J. Colburn in the July, 1967 Journal of the Royal Aeronautical Society. However, the operational and control environment for jet engine operation differs considerably from that for industrial gas turbines.
The aforereferenced U.S. Pat. No. 82,470, assigned to the present assignee, presents an improved system and method for operating a gas turbine with a digital computer control system. In this system, one or more turbine-generator plants are operated by a hybrid digital computer control system, wherein logic macro instructions are employed in programming the computer for logic operations of the control system.
The computerized gas turbine control as disclosed in U.S. Pat. No. 82,470 has been highly successful in providing control capability and flexibility of control options that had not previously been incorporated into an all hardware type system. However, while the computerized, or software control system provides substantial advantages due to its logic performing capability, historical data storage and diagnostic programs, it also has a number of shortcomings. The analog input system is a complex multiplexing arrangement requiring sharing of the scan time by the variables which must be scanned or read "independently". In the system disclosed, there is a scanning rate of 30 per second, meaning that 30 input variables per second can be read, imposing a limitation on the ability of the system to respond rapidly to a given input variable when program running time is also added to the delay. In addition, the computer system itself incorporates elaborate techniques of self-diagnosis of failure, which can result in turbine shutdowns when the computer has determined that something has failed within the central processor, input-output, or peripheral hardware. It is most difficult for the computer to determine whether the failure is of a sufficiently critical nature to require shutdown. In fact, it has been found that failures in the analog input-output system may not be readily differentiated, leaving the computer no choice but to shut down the entire turbine system for a failure which may not justify loss of load availability. Since all monitoring and protection paths are channeled through a central processor, a self-determination of failure in the central processor, analog input multiplexing or output system by the computer controller necessitates blocking off all channels, such that complete system shutdown is required. Furthermore, even during normal operation, the computerized system provides low visibility with respect to the health of the control system. The essential intermixing of the control paths through the central processor makes it difficult for the operator to obtain information as to the mode of control at any moment, or to obtain quantitative information as to the relative magnitudes of the different control signals. In short, the increased flexibility of the software system is achieved at the expense of operator visibility such as permits optimum maintenance procedures. Accordingly, there is a great need in the art for a turbine system having a control with the logic capability of a digital system, but retaining the advantages which are inherent in simpler designs.
Furthermore, in much of the prior art, little or no consideration is given to backup starting or running controls for the computer implemented automatic turbine control system when such control system must be shut down for maintenance or else experiences a failure. This is due primarily to the inclusion of multiple control loops in the automatic control system itself, which multiplicity serves to some degree as backup protection, especially where the control loops are embodied in relatively independent hardware paths as, for example, in an analog electropneumatic controller.
In one instance, a manual backup control for a steam turbine has been provided; see U.S. Pat. No. 3,552,872 which issued on Jan. 5, 1971 to T. Giras and W. Barnes, Jr. However, the operating environment and characteristics of steam and gas turbines differ dramatically. In addition, the necessary interface between primary and backup controllers in steam and gas turbines is divergent. Consequently, while the teachings of this patent might suggest the possible use of a backup control for a gas turbine control system, the operational and characteristic differences severely limit, if not negate completely, such utilization.
The local maintenance controller as described herein incoporates novel features which are specifically designed to meet and fully satisfy the above-stated requirements for a gas turbine power plant backup controller and provides such backup control in a manner and to a degree not heretofore known or available.
In referencing prior art publications or patents as background herein, no representation is made or intended that the cited subject matter is the most pertinent known.